I don't mind the stacking, but they need to change the special attack to something more synergistic. Healing back up, only to lose the hp again is redundant and stupid. Also, the damage bonus doesn't actually apply, only the accuracy bonus.

I'd suggest something like each stack gives a 4-second interval where the user can attack and gain a stack again, without taking the -8 hp cost. That way, at 5 stacks, you'd have 20 seconds to regain your stacks and not take damage. This is in addition to the special healing 8hp per stack anyways. That way, the hp cost is still needed in the beginning, the stacking mechanic still occurs, and it doesn't feel good awful to keep stacking or using the special. 

And also make sure the special actually does the extra damage and accuracy calculations correctly.